I use every food as if it were "supplementary", meaning I feed a variety of foods on a fairly equal basis. Freeze dried tubifex is one of the foods that I feed. A lot of people use one certain food (usually flake) as a staple, and considering that most modern flake is manufactured for such purposes it is probably okay to do. I don't know how the average freeze-dried tubifex would compare to the average flake food nutritionally, but comparing the labels would give an idea.
IMO a varied diet is a good thing, as long as you provide the right kinds of foods for the specific fish you are feeding.